Nanogels for delayed gelation

ABSTRACT

The instant application relates to nanogels or compositions that hold multivalent metal ions until some level of nanogel degradation has occurred, then slowly release the multivalent metal ions for gelation with carboxylate containing polymers. Compositions comprising such nanogels, together with polymers that can be crosslinked with multivalent metal ions, allow the deployment of such mixtures in various applications, and greatly increased gelation times.

PRIOR RELATED APPLICATIONS

This application is a divisional of allowed U.S. patent application Ser.No. 16/938,368, filed Jul. 24, 2022 (published as US20200354625) whichis a division of U.S. patent application Ser. No. 15/708,034 filed onSep. 18, 2017 (now U.S. patent Ser. No. 10/752,826), which is acontinuation of U.S. patent application Ser. No. 14/143,169 filed Dec.30, 2013 (now U.S. Pat. No. 9,796,909), which claims the benefit ofprovisional U.S. Patent Application No. 61/754,060, filed on Jan. 18,2013. Each application is incorporated by reference in its entiretyherein for all purposes.

FEDERALLY SPONSORED RESEARCH STATEMENT

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

FIELD OF THE DISCLOSURE

The disclosure relates to delayed gelling agents that can be deployed inoil and gas reservoirs, as well as any other agriculture, remediation,mining or other industrial uses.

BACKGROUND OF THE DISCLOSURE

The challenge for all oil and gas companies is to produce as much oil ascommercially feasible, leaving as little oil as possible trapped insidethe reservoir. During the primary recovery stage, reservoir drive comesfrom a number of natural mechanisms. These include natural water pushingoil towards the well, expansion of the natural gas at the top of thereservoir, expansion of gas initially dissolved in the crude oil, andgravity drainage resulting from the movement of oil within the reservoirfrom the upper regions to lower regions where the wells are located.Recovery factor during the primary recovery stage is typically about5-15% under such natural drive mechanisms.

Over the lifetime of a well, however, the pressure will eventually fall,and at some point there will be insufficient underground pressure toforce the oil to the surface. Once the natural reservoir drivediminishes, secondary and tertiary recovery methods are applied tofurther increase recovery. Secondary recovery methods rely on the supplyof external energy into the reservoir in the form of injecting fluids toincrease reservoir pressure, hence replacing or increasing the naturalreservoir drive with an artificial drive. In addition, pumps, such asbeam pumps, pumps with gas lift and electrical submersible pumps (ESPs),can be used to bring the oil to the surface. Secondary recoverytechniques include increasing reservoir pressure by water injection, CO₂injection, natural gas reinjection, and miscible injection, the mostcommon of which is probably water injection. Typical recovery factorfrom water-flood operations is about 30%, depending on the properties ofoil and the characteristics of the reservoir rock. On average, therecovery factor after primary and secondary oil recovery operations isbetween 35 and 45%.

While secondary recovery techniques are fairly effective, the existenceof fractures and regions of highly porous or permeable rock reduce theirpotential effectiveness. Any gas or liquid that is injected into a well,will naturally travel the least restrictive route, thus bypassing someof the oil in the less porous or permeable regions. Thus, the overalleffectiveness of the sweep is reduced by these so-called “thief zones,”which channel injection fluid directly to production wells.

In such cases, polymers are injected into the thief zones in order toblock these zones, thus diverting the subsequent injection fluids topush previously unswept oil towards the production wells. Gels have beenapplied in enhanced oil recovery to improve the sweep efficiency,prolong the life of an oil well and maximize the recoverable oil amountby placing the gelants deep into the reservoir and blocking thehigh-permeability channels.

One of the difficulties involving the use of gelants to block thiefzones, though, is increasing the viscosity of the gelants. Viscousgelants are difficult to pump and can shear during pumping, making itmore difficult and expensive to get the viscous solutions deep into thereservoir, yet high viscosity is needed to block the thief zones. Forthis reason, there is considerable effort directed to delaying thefurther crosslinking of polymers until they have already penetrated deepinto the reservoir.

Among the polymers used for such purposes, partially hydrolyzedpolyacrylamide (HPAM) cross linked with Cr (III) gels have been widelyused for water shutoff and sweep improvement in field applications.Other metal ions that can further crosslink polymers containing anionicsites include zirconium, titanium, iron, aluminum and combinationsthereof.

Usually these metallic ions crosslink gellable polymers through theinteraction with e.g., the carboxylate groups of the polymer molecules.Generally, the gellable polymers used such as, for example,polyacrylamide are of high molecular weight and contain high degrees ofhydrolysis, i.e., contain 10-30 mole % carboxylate groups. However,these high molecular weight and/or high mole % carboxylategroup-containing polymers gel almost instantly in the presence of theabove-described multivalent metallic compounds. Such fast gelation raterenders the application of gelling compositions containing thesepolymers and multivalent metallic compounds not useful in many oil-fieldapplications such as, for example, water shut-offs and permeabilityreductions, since the gelant crosslinks before it has had a chance topenetrate the reservoir, thus stopping its flow. Furthermore, theresulting gels typically synerese heavily in most oil-field brines,depending on reservoir temperature and the divalent cation content ofthe brine.

Many efforts have been directed to delaying the gelation of suchpolymers by adding a gelation delaying agent to the compositions. Theuse of ligands complexed with multivalent cations such as Al(III),Cr(III), Ti(IV) and Zr(IV) to crosslink partially hydrolyzedpolyacrylamides (HPAM) has been a common practice to slow the rate ofreactions of these cations with HPAM. The presence of ligands such asacetate, citrate, propionate, malonate, etc., which bind to multivalentcations, inhibit rapid interaction of the multivalent cations with thenegative sites of HPAM to produce gels, thus delaying the rate ofgelation.

An extensive study (Albonico 1993) performed on evaluating variousretarding ligands, ranked the effectiveness of hydroxycarboxylates,dicarboxylates and aminocarboxylates on retarding the gelation rate ofCr(III) with HPAM solutions. This study showed that malonate ions are 33times slower than acetate to gel 0.5% HE-100, a copolymer of acrylamideand sodium AMPS, at 120° C. This study ranked ascorbate to be 51 timesslower than acetate under the same conditions. The authors furthertested the effectiveness of various ligands in propagation of Cr(III)ions in both sandstone and carbonate formations. They concluded thatmalonate ions are most effective in promoting propagation of Cr(III) inporous media, preventing precipitation and thus retention of Cr(III).

While the rates of gelation of HPAM with complexes of multivalentcations are slower than for un-complexed multivalent cations, they arestill not slow enough. Extensive gelation tests with complexes ofmultivalent cations with HPAM indicate formation of non-flowing gelswithin a few hours, not long enough for deep placement of the gelantsbefore reaching the non-flowing stage. Additionally, the integrity ofthe stabilized package due to chromatographic separation might hindertheir effectiveness of such systems in treating high permeabilitytargets deep in porous media matrix.

Extending the gelation times from a few hours to days or weeks, istherefore, highly desirable for the placement of the gelants deep inmatrix target zones. Further, a less toxic package that is very stablein various brines and at typical reservoir temperatures would also bedesirable, since the increased stability will allow deeper deployment.

SUMMARY OF THE DISCLOSURE

The present disclosure teaches the formation of degradable nanogelsproduced with multivalent cations. Such degradable nanogels can beinjected along with another anionic polymers, such as HPAM, into thetarget zones, at which time the labile bonds in the nanogel break,slowly releasing the multivalent cation, and allowing it to react withthe anionic sites of the second anionic polymer to produce gels andblock high permeability channels.

A large variety of novel polymeric nanogels are shown herein, includingthose produced with the reaction of a complex multivalent cation, suchas Cr(III) acetate, and a carboxylated polymer, such as polyvinylalcohol succinate (PVA succinate) with PVA molecular weights of 6 kDaand 25 kDa (FIGS. 1 and 2), as well as similar nanogels made withzirconium ions, and small particle nanogels made with inverse emulsiontechniques. Nanogels made with carboxylated polypeptides, such aspolyasparate, are also shown.

All nanogels are shown to significantly delay gelling of a variety ofpolymers, at varying temperatures, and thus are suitable for reservoiruse. Nanogels produced with PVA succinate or polyaspartate and Cr(III)chloride which is cheaper than Cr(III) acetate or Cr (III) propionatewere also produced. Such nanogels produce gels with carboxylatecontaining polymers such as HPAM and B29 at very slow rates to place thegelants deep into the target zones before setting into immobile gels.Addition of Cr(III) chloride to HPAM or B29 polymers under similarconditions result in instant gelation or precipitation.

In the high temperatures of the reservoir environment, the carboxylatesor esters or amides of the nanogel break or degrade, thus releasing themultivalent metal ions, which are thus free to crosslink a secondpolymer also containing pendant carboxylates, thus gelling said polymerin situ.

We have exemplified the degradable nanogels herein using PVA succinateand polyasparate, but it is likely that many other molecules withsimilar chemistry could be used. Thus, any dicarboxylate that can bedehydrated to form an anhydride can be conjugated to PVA using similarchemical reactions. Thus, maleic anhydride (cis-butenedioic anhydride),is expected to be substitutable in the invention, as are glutaricanhydride, phthalic anhydride, etc. Further, any di- or tricarboxylatecan be polymerized and used, based on our success with polyaspartate.

The two main requirements of the degradable nanogel are 1) that itcontain pendant carboxylates or other anions for complexing the metalion, and 2) that it degrades in situ, so as to release the metal ions,for further crosslinking of the reservoir injection polymer, which alsocontains anionic groups, often carboxylates. Thus, the nanogel will beless chemically stable under reservoir conditions than the injectionpolymer, which is selected to be stable under the same conditions.Pendant carboxylate groups, esters or amides may all provide thechemistry for the needed degradation.

Further, though we used PVA as a base polymer to add the succinate to,any polymer containing double bonds (such as vinyl, allyl, styrene,acrylamide, etc.). can be conjugated to e.g., succinate anhydride.

Examples of some monomers and their synthesis are shown in FIG. 3including: N-hydroxylmethyl acrylamide (NHMA) succinate, allyl alcoholsuccinate and allylamine succinate, each of which may be synthesizedthrough the reaction of NHMA, allyl alcohol and allylamine with e.g.,succinic anhydride (or malic anhydride) to make polymers suitable forpreparing the degradable nanogels of the invention.

In principle, all polymers containing temporary carboxyl groups can beused to make multivalent metal ion-loaded particles for gelation delay.“Temporary carboxyl groups” or “releasable carboxyl groups” means thatcarboxyl groups can be removed from polymer chains due to the breakingof a bond or bonds between carboxyl groups and the polymer chain, or bybreaking the polymer chain itself (such as polycarbonate containingcarboxyl groups or peptides contain carboxyl groups, such aspolyaspartic acid and polyglutamic acid, as shown FIG. 4). The carboxylgroups may be conjugated to polymer chains through ester bonds, amidebonds, and the like. The bond type will affect release rate of metalions. For example, amide bond breaking is typically more difficult thanester bond, so the metal ions released from particles where amide bondsare used may be much slower than particles where ester bonds are used.

Exemplary metal ion containing nanogels are listed in Table 1.

TABLE 1 Exemplary nanogels Ratio of Metal ion Cr(III) COOH:metal,Nanogel Polymer or Zr(IV) loading mole/mole Method Cr-Nanogel-27 135 mgPVAS-25 1500 ppm Cr 6:1 Solution Cr-Nanogel-28 101 mg PVAS-25 1500 ppmCr 4.5:1   Solution Cr-Nanogel-30 68 mg PVAS-25 1500 ppm Cr 3:1 SolutionCr-Nanogel-29 143 mg PVAS-6 1500 ppm Cr 6:1 Solution Cr-Nanogel-32 794mg PVAS-6 3600 ppm Cr 6:1 Inverse emulsion Cr-Nanogel-33* 368 mg PVAS-253600 ppm Cr 6:1 Inverse emulsion Zr-Nanogel-43 328 ppm PVAS-6 4463 ppmZr 6:1 Solution Cr-PAsp 921 ppm PAsp acid 6837 ppm Cr 7:1 InverseNanogel-2* emulsion *Nanogels were prepared in NaOH solution

The term “carboxylate-containing polymer” used herein in a delayedgelling composition refers to, unless otherwise indicated, a polymerthat contains at least one free carboxylic acid group or a carboxylategroup in which the proton of the carboxylic acid is substituted with anammonium ion, an alkali metal ion, an alkaline earth metal ion, orcombinations of any two or more thereof, such that the pendantcarboxylate groups can be crosslinked with a multivalent metal ion, thusforming a gel.

According to the present disclosure, the molecular weight of thecarboxylate-containing polymers is generally at least about 10,000 Daand less than about 25,000,000 Da, preferably less than about 20,000,000Da.

The mole percent % of the carboxylate group in carboxylate-containingpolymers, such as partially hydrolyzed polyacrylamides (HPAM), isgenerally in the range of from about 0.01 to less than about 45,preferably about 0.1 to less than about 25, more preferably about 0.1 toless than about 15, even more preferably about 0.1 less than about 10,and most preferably 0.2 to 10 mole %.

According to the present invention, the gelation time is defined as thetime when the viscosity of the gel solution increases abruptly to avalue greater than 1000 cP (100% scales) at a shear rate of 2.25 s⁻¹.

The gelation time is generally longer than about 3 days, 5 days, a week,10 days, 30 days or more, depending on temperature, nanogel compositionand crosslinkable polymer composition and concentration.

Carboxylate-containing polymers suitable for use in this invention arethose capable of gelling in the presence of a crosslinking agent suchas, chromium or zirconium, and are preferably stable at reservoirconditions. Polymers suitable for use in this invention, include, butare not limited to, polysaccharides, such as carboxylatedpolysaccharides or carboxylated guar, cellulose ethers, such ascarboxymethyl cellulose, and acrylamide-containing polymers.

Suitable acrylamide-containing polymers that also contain pendantcarboxylate groups are disclosed in U.S. Pat. No. 3,749,172 (hereinincorporated by reference in its entirety).

Particularly preferred acrylamide-containing polymers are the so-calledpartially hydrolyzed polyacrylamides possessing pendant carboxylategroups via which crosslinking can take place. Thermally stablecarboxylate-containing polymers of acrylamide, such as terpolymers ofN-vinyl-2-pyrrolidone and acrylamide and sodium acrylate; tetrapolymersof sodium-2-acrylamido-2-methylpropanesulfonate, acrylamide,N-vinyl-2-pyrrolidone and sodium acrylate; and terpolymers ofsodium-2-acrylamido-2-methylpropanesulfonate and acrylamide and sodiumacrylate; terpolymers of N,N dimethylacrylamide and acrylamide andsodium acrylate; and combinations of any two or more thereof, areparticularly preferred for applications in high salinity environments atelevated temperatures for stability. Selected carboxylate-containingterpolymers also are useful in the present process, such astetrapolymers derived from acrylamide, sodium acrylate, andN-vinyl-2-pyrrolidone and N,N-dimethylacrylamide co-monomers with lesseramounts of monomers such as vinyl acetate, vinylpyridine, styrene,methyl methacrylate, and other polymers containing acrylate groups.

Any crosslinking agents, such as e.g. a multivalent metallic compound,that are substantially suspendable in the liquid component of thecomposition and are capable of crosslinking the carboxylate-containingpolymer in the hydrocarbon-bearing formations can be used in the processof the present invention.

Suitable multivalent metal ions include chromium, zirconium, titanium,aluminum and the like. The metal ions can also be complexed with aligand, such as acetate, propionate, malonate, citrate and the like.

The presently preferred multivalent metallic compound is selected fromthe group consisting of zirconium compounds, titanium compounds,aluminum compounds, iron compounds, chromium compounds, such as Cr(III)chloride Cr(III) acetate, Cr(III) propionate, and combinations of anytwo or more thereof. Examples of suitable multivalent metallic compoundsinclude, but are not limited to, sodium zirconium lactate, potassiumzirconium lactate, ammonium zirconium lactate, ammonium zirconiumcarbonate, sodium zirconium carbonate, potassium zirconium carbonate,ammonium zirconium fluoride, ammonium zirconium chloride, zirconiumammonium citrate, zirconium chloride,tetrakis(triethanolamine)zirconate, zirconium carbonate, zirconylammonium carbonate, ammonium titanium carbonate, titanium chloride,titanium carbonate, ammonium titanium chloride, and combinationsthereof. These compounds are commercially available. The presently mostpreferred crosslinking agents are sodium zirconium lactate and ammoniumzirconium carbonate, chromium acetate, chromium propionate, chromiummalonate, chromium (III) chloride, etc.

The concentration of crosslinking agent used in the present inventiondepends largely on the concentrations of polymer in the composition andthe desired gelation delay. Lower concentrations of polymer, e.g.,require lower concentrations of the crosslinking agent. Further, it hasbeen found that for a given concentration of polymer, increasing theconcentration of crosslinking agent generally substantially decreasesthe time of gelation (increases the gelation rate). The concentration ofcrosslinking agent in the injected slug varies generally over the broadrange of about 1 mg/l (ppm) to about 1,000 ppm, preferably over therange of about 1 ppm to about 500 ppm, and most preferably 1 ppm to 200ppm based on Cr(III).

Any suitable procedures for preparing the aqueous admixtures of thegellable polymers, degradable nanogels, and liquid can be used. Some ofthe polymers can require particular mixing conditions, such as slowaddition of finely powdered polymer into a vortex of stirred brine,alcohol prewetting, and protection from air (oxygen), preparation ofstock solutions from fresh rather than salt water, as is known for suchpolymers.

As used herein, “ppm” refers to parts per million on a weight per weightbasis.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims or the specification means one or more thanone, unless the context dictates otherwise.

The term “about” means the stated value plus or minus the margin oferror of measurement or plus or minus 10% if no method of measurement isindicated.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or if thealternatives are mutually exclusive.

The terms “comprise”, “have”, “include” and “contain” (and theirvariants) are open-ended linking verbs and allow the addition of otherelements when used in a claim.

The phrase “consisting of” is closed, and excludes all additionalelements.

The phrase “consisting essentially of” excludes additional materialelements, but allows the inclusions of non-material elements that do notsubstantially change the nature of the invention.

The following abbreviations are used herein:

ABBREVIA- TION TERM B29 a swellable microparticle copolymer ofacrylamide and sodium acrylate crosslinked with poly(ethylene glycol)(258) diacrylate and methylene bisacrylamide, swells or expands whenlabile diacrylate bonds break. CrAc Chromium (III) acetate hydroxide(CH₃COO)₇Cr₃(OH)₂ Da Dalton HPAM Partially hydrolyzed PolyacrylamideNHMA N-hydroxylmethyl acrylamide NMP N-methyl-2-pyrrolidone PAspPolyaspartate PSH polyoxyethylene sorbitol hexaoleate PVA polyvinylalcohol PVAM PVA malate PVAS PVA succinate PVAS-25 PVA succinate 25 KDaPVAS-6 PVA succinate 6 kDa RO Reverse osmosis TEA Triethylamine ZrLaZirconium (IV) Lactate

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the synthesis of PVA succinate.

FIG. 2 shows the basic concept of multivalent metal ion-loaded nanogelfor gelation.

FIG. 3 shows synthesis of NHMA succinate, allyl alcohol succinate andallylamine succinate.

FIG. 4 displays examples of temporary carboxyl groups, includingpolycarbonate-containing carboxyl groups, polyaspartate andpolyglutamate, that could be used to make degradable nanogels pursuantto this disclosure.

FIG. 5 shows the gelation of 0.5% HPAM in Brine A crosslinked with 100ppm Cr(III) as Cr-nanogel-27, taking 12 days of aging to begin gellingat 85° C. This polymer would set to a gel within a few hours at 85° C.if the crosslinker was only Cr(III)-acetate. Using Cr-nanogel-28 andCr-nanogel-30, gelation occurred at 10 days at 85° C. Therefore, thehighest ratio of COOH/Cr (Cr-Nanogel-27) took the longest time to gel.This is because more COOH groups bind the Cr ions stronger, making themless available for crosslinking HPAM.

FIG. 6 shows the gelation of 0.5% HPAM in Brine A crosslinked withCr-nanogel-27 (PVAS-25-6:1) and Cr-nanogel-29 (PVAS-6-6:1) containing100 ppm Cr(III) aged at 85° C. As this plot shows, it took over 10 daysof aging at 85° C. for this system to begin the gelation process. Thetwo nanogels are made with different molecular weight polymers, but aresomewhat different based on the vinyl acetate content as well, making itdifficult to provide conclusive statements. However, the two polymershad the similar delays, indicating that the molecular weight of thepolymer used to make the nanogel had little effect, at least at theseconditions.

FIG. 7 shows the gelation results for a solution of 0.5% HPAM in Brine Acrosslinked with 100 ppm Cr(III) in the form of Cr-nanogel-32(PVAS-6-6:1 made by inverse emulsion) described below. This gelant beganto gel in about 10 days of aging at 85° C. The same gelant solutionbegan to gel after 10 weeks of aging at 65° C. Typically reaction ratedouble for every 10° C. rise in temperature. Thus, 85° C. is expected beabout four times faster than 65° C.

FIG. 8 shows the viscosity versus aging time for gelation of 0.5% HPAMin Brine A exposed to 100 ppm Cr in the form of Cr-nanogel-33(PVAS-25-6:1, made by inverse emulsion) aged at 65° C. and 85° C. Thisgelant required about 65 days of aging at 65° C. and about 5 days at 85°C. to begin gelling. In this instance, the Cr-nanogel-33 was made inNaOH solution, which may have affected gelation time.

FIG. 9 summarizes gelation tests results for 0.5% B29 polymericmicroparticle in Brine A exposed to 100 ppm Cr(III) in the form ofCr-nanogel-32 (PVAS-6-6:1 inverse emulsion) aged at 65° C. and 85° C.While the gelant aged at 65° C. took over 7 weeks of aging to begingelling, the gelant aged at 85° C. began to gel in about 5 days ofaging.

FIG. 10 shows viscosity versus aging time for 0.5% B29 polymericmicroparticle in Brine A exposed to 100 ppm Cr(III) in the form ofCr-nanogel-33 (PVAS-25-6:1, inverse emulsion) aged at 65° C. and 85° C.While the gelant aged at 85° C. began to gel in about 5 days of aging,the gelant aged at 65° C. took over 5 weeks of aging to exhibit asubstantial increase in viscosity.

FIG. 11 shows viscosity versus aging time for 0.5% HPAM withZr-nanogel-43 [Zr(IV) concentration of 120 ppm] in Synthetic Brine Awithout NaHCO₃ at 65 and 88° C. While the PVAS-6-6:1 gelant aged at 65°C. took over around three weeks of aging to begin gelling, the gelantaged at 85° C. began to gel in about 2-5 days of aging.

FIG. 12 shows viscosity versus aging time for 0.5% HPAM with 100 ppmCr(III) as Cr-PAsp nanogel-2 in Synthetic Brine B at 88° C. and 106° C.While the gelant aged at 88° C. took over around 34 days of aging tobegin gelling the gelant aged at 106° C. began to gel in about 10 daysof aging.

FIG. 13 shows viscosity versus aging time for 0.5% B29 with 100 ppmCr(III) as Cr-PAsp nanogel-2 in Synthetic Brine B at 88 and 106° C.While the gelant aged at 88° C. took over around 28 days of aging tobegin gelling the gelant aged at 106° C. began to gel in about 8 days ofaging.

FIG. 14 shows viscosity versus aging time for 0.5% B29 with 100 ppmCr(III) as CrCl₃-PVAS in Synthetic Brine A at 65 and 85° C. While thegelant aged at 65° C. took over around 62 days of aging to begingelling, the gelant aged at 85° C. began to gel in about 9 days ofaging.

FIG. 15 shows viscosity versus aging time for 0.5% B29 with 100 ppmCr(III) as CrCl₃-PAsp (CrCl₃-PAsp-1 and CrCl₃-PAsp-2 are the sameformulation, used to prove the reproducibility of gelation delay) inSynthetic Brine A at 100 and 120° C. While the gelant aged at 120° C.took about 1 day of aging to begin gelling the gelant aged at 100° C.began to gel in about 3-4 days of aging.

FIG. 16 shows viscosity versus aging time for 0.5% B29 with 100 ppmCr(III) as CrCl₃-PAsp (CrCl₃-PAsp-1 and CrCl₃-PAsp-2 are the sameformulation, used to prove the reproducibility of gelation delay) inSynthetic Brine A at 85° C. The gelant took over around 39 days of agingto begin gelling aged at 85° C.

FIG. 17 shows viscosity versus aging time for 0.5% HPAM with 100 ppmCr(III) as CrCl₃-PAsp (CrCl₃-PAsp-1 and CrCl₃-PAsp-2 are the sameformulation, used to prove the reproducibility of gelation delay) inSynthetic Brine A at 100 and 120° C. While the gelant aged at 120° C.took about 1 days of aging to begin gelling the gelant aged at 100° C.began to gel in about 2-3 days of aging.

FIG. 18 shows viscosity versus aging time for 0.5% HPAM with 100 ppmCr(III) as CrCl₃-PAsp (CrCl₃-PAsp-1 and CrCl₃-PAsp-2 are the sameformulation, used to prove the reproducibility of gelation delay) inSynthetic Brine A at 85° C. The gelant took over around 30 days of agingto begin gelling aged at 85° C.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The disclosure provides novel compositions and methods, including anyone or more of the following embodiments:

A degradable composition is provided, comprising a polymer having labileor releasable carboxylate groups complexed with a multivalent metal ion,said degradable composition lasting at least 5 days at 85° C. in a brinesolution having 23 g/l NaCl, and thereafter degrading said compositionand releasing said multivalent metal ion. Degradation is thus to beascertained by release of the multivalent metal ion, which isascertained by a second polymer gelling, as shown in the viscosityincrease experiments described herein.

In preferred embodiments, the composition is a nanogel, but this is notan absolute requirement. Nanogels include particles of less than onemicron, preferably 200-500, and most preferably 400 nm.

The polymer used to form nanogel can be made from monomers selected fromthe group of vinyl, allyl, styrene, and acrylamide monomers and theirderivatives, or any polysaccharide, conjugated with a dicarboxylate orhaving naturally appended carboxylate groups. Any dicarboxylate (ortricarboxylate) can be used, including citrate, succinate, aspartate,glutamate, malate, oxalate, malonate, glutarate, adipate, pimelate, andthe like, or a derivative thereof.

In some embodiments, the degradable nanogel having carboxylate groups isa polymer or copolymer of citrate, succinate, aspartate, glutamate,malate, oxalate, malonate, glutarate, adipate, pimelate, carbonate, andthe like, or derivatives thereof.

In some preferred embodiments, the nanogel comprises polyvinyl alcohol(PVA) succinate, N-hydroxylmethyl acrylamide (NHMA) succinate, allylalcohol succinate and allylamine succinate, PVA malate, NHMA malate,allyl alcohol malate or allylamine malate. In other embodiments, thepolymer is polyaspartate or polyglutamate, or the like.

The multivalent metal ion is any such ion whose presentation needs bedelayed, and for reservoir use for tertiary crosslinking includeschromium, zirconium, iron, aluminum, and titanium.

In some embodiments, the nanogel is PVA succinate and chromium orzirconium. In others, it is PVA malate and chromium or zirconium. In yetothers, it is polyaspartate and chromium or zirconium, or polyglutamateand chromium or zirconium. Other exemplary nanogels are selected fromTable 1.

Preferably, the carboxylate group to multivalent metal ion molar ratiois increased to delay release of the multivalent cation, and preferredembodiments include ratios from 3:1 to 15:1, or even 20:1. A 6:1 ratiowas useful for the delays shown herein, but higher ratios may bepreferred for hotter reservoirs.

The multivalent metal ion is present at amounts needed for theapplication, but in reservoir applications a lower amount is preferredas being more cost effective. Amounts thus range from 10-10,000 ppm or50-5000 ppm, or about 1-200 ppm, such ppm given as the finalweight/weight basis of the complete injection fluid.

Other embodiments provide a degradable nanogel comprising PVA succinateor PVA malate or polyasparate complexed with multivalent metal ioncomprising chromium, zirconium, iron, aluminum, titanium or combinationsthereof, said degradable nanogel lasting at least 5 days to 10 days at85° C. in a brine solution having 23 g/l NaCl, and thereafter degradingsaid nanogel and releasing said multivalent metal ion. Other degradablenanogels last at least 10 days at 85° C., and/or at least 30 days at 65°C.

Delayed gelling compositions are also provided, comprising anydegradable nanogel herein described, admixed with an injection fluidadmixed with a carboxylate containing polymer.

Any carboxylate containing polymer can be used in the injection fluid,provided such polymer can be crosslinked with the metal ion in thenanogel. Such polymers include, e.g., partially hydrolyzedpolyacrylamide, copolymers of N-vinyl-2-pyrrolidone and sodium acrylate,tetrapolymers of sodium-2-acrylamido-2-methylpropanesulfonate,acrylamide and N-vinyl-2-pyrrolidone and sodium acrylate; and copolymersof sodium-2-acrylamido-2-methylpropanesulfonate and sodium acrylate; andcombinations thereof.

Improved methods of sweeping for oil are also provided. In oneembodiment, wherein prior methods required blocking thief zones with apolymer, and sweeping a reservoir for oil, the improved methodcomprising injecting any delayed gelling composition herein describedinto a reservoir, aging said composition until the viscosity increases,and sweeping the reservoir for oil.

Improved methods of producing oil or gas are also provided, priormethods requiring injecting a polymer into a reservoir and producing oilor gas, the improved methods comprising injecting any of the delayedgelling compositions herein described into a reservoir, aging saidcomposition until the viscosity increases, and producing said oil orgas.

The following experiments were performed to synthesize multivalent metalion loaded degradable nanogels for use as delayed crosslinking agents toproduce gels with anionic polymers deep into oil-bearing formations.

PVA succinate, 6 k. A representative poly(vinyl alcohol succinate),herein referred to as PVA succinate, 6 kDa was prepared through thereaction of poly(vinyl alcohol, Mw 6 k, 80 mol % degree of hydrolysis),(PVA-6) and succinic anhydride using triethylamine (TEA) as catalyst inN-methyl-2-pyrrolidone (NMP) as solvent. First, 10 g PVA-6 was dissolvedin 120 g NMP at 80° C. while stirring. Second, the solution wasmaintained at 60° C. reaction temperature, and 15 g TEA and 15 gsuccinic anhydride in 40 g NMP were added while stirring. After 22 hoursat 60° C., PVA succinate 6 k (PVAS-6) was purified by precipitation inether and dried under vacuum. FIG. 1 shows the chemical composition ofPVAS.

PVA succinate 25K. A representative poly(vinyl alcohol succinate),herein referred to as PVA succinate, 25 kDa was prepared through thereaction of poly(vinyl alcohol, Mw 25 k, 88 mol % degree of hydrolysis)(PVA-25) and succinic anhydride using TEA as catalyst in NMP as solvent.First, 10 g PVA-25 was dissolved in 135 g NMP at 80° C. while stirring.Second, the solution was maintained at 60° C. reaction temperature, and18 g TEA and 18 g succinic anhydride in 45 g NMP were added whilestirring. After 22 hours at 60° C., PVAS-25 was purified byprecipitation in ether and dried under vacuum.

Cr-Nanogel-27, 28 and 30 with PVAS-25 and CrAc. A representativeCr(III)-loaded nanogel herein referred to as Cr-nanogel-27 was preparedthrough mixing PVAS-25 with Cr(III) as CrAc in Reverse Osmosis (RO)water while stirring. 135 mg PVAS-25 was dissolved in 4.84 g RO waterand 29 mg Cr-Acetate was added into the above solution while stirring.The carboxyl groups/Cr (III) molar ratio is 6:1. Cr(III) loading inCr-nanogel-27 was around 1500 ppm. Cr-nanogel-28 and Cr-nanogel-30having lower carboxyl groups/Cr (III) molar ratio were prepared usingthe same procedure. Detailed information regarding these nanogels islisted in Table 1.

Cr-Nanogel-29 with PVAS-6 and CrAc. A representative Cr(III)-loadednanogel herein referred to as Cr-nanogel-29 was prepared through mixingPVAS-6 with Cr(III) as CrAc in RO water while stirring. 143 mg PVAS-6was dissolved in 4.83 g RO water and 29 mg CrAc was added into the abovesolution while stirring. The carboxyl groups/Cr(III) molar ratio was6:1. Cr(III) loading in Cr-nanogel-29 is around 1500 ppm. Detailedinformation regarding Cr-nanogel-29 is listed in Table 1.

Cr-Nanogel-31 with PVAS-6 and CrAc. Cr-Nanogel-31 was made of PVAS-6 andCr-acetate in Synthetic Brine A for the compatibility test with brine.Cr-Nanogel-31 looked homogeneous and its gelation delay with HPAM wassimilar to other nanogels (data not shown), but a lot of bubblesappeared during dissolving PVA succinate in Brine A due to CO₂ releaseresulting from reaction of PVA succinate carboxyl groups with NaHCO₃ inBrine A.

Cr-nanogel-32 with PVAS-6 and CrAc by inverse-emulsion. A representativeCr(III)-loaded nanogel herein referred to as Cr-nanogel-32 was preparedusing PVAS-6 and CrAc by inverse-emulsion in order to prepare small sizeparticles. In such process, an aqueous mixture containing 794 mg PVAS-6,158 mg CrAc and 6.0 g RO water as the dispersed phase and an oil mixtureof 3.5 g kerosene, 557 mg Span 83 and 313 mg polyoxyethylene sorbitolhexaoleate (PSH) as a continuous phase were prepared. Theinverse-emulsion was prepared by mixing the aqueous phase and the oilphase, followed by rapid homogenization using a sonicator. The carboxylgroups/Cr(III) molar ratio was 6:1. Cr(III) loading in Cr-nanogel-32 wasaround 3600 ppm. The mean particle size, measured in RO water by dynamiclight scattering experiments employing a ZetaPALS zeta potentialanalyzer (Brookhaven Instruments Corp.), was around 400 nm. Detailedinformation regarding Cr-nanogel-32 is listed in Table 1.

Cr-Nanogel-33 with PVA succinate, 25 k and CrAc by inverse-emulsion. Arepresentative Cr(III)-loaded nanogel herein referred to asCr-nanogel-33 was prepared using PVAS-25 and CrAc by inverse-emulsion.In order to increase the solubility and ionization degree of PVAsuccinate, the partial carboxyl groups of PVA succinate were transformedto sodium carboxylate. In such process, an aqueous mixture containing368 mg PVAS-25, 62 mg NaOH, 79 mg CrAc and 3.0 g RO water as thedispersed phase and an oil mixture of 1.7 g kerosene, 279 mg Span 83 and157 mg PSH as continuous phase were prepared. The inverse-emulsion wasprepared by mixing the aqueous phase and the oil phase, followed byrapid homogenization using a sonicator. The carboxyl groups/Cr(III)molar ratio was 6:1. Cr(III) loading in Cr-nanogel-33 was around 3600ppm. The mean particle size was around 400 nm. Detailed informationregarding Cr-nanogel-33 is listed in Table 1.

Zr-Nanogel-43 with PVA succinate, 6 k and ZrLa. In order to comparechromium ions against zirconium ions, a representative Zr(IV)-loadednanogel herein referred to as Zr-nanogel-43 was prepared through mixingPVAS-6 with Zr(IV) as Zr-lactate (ZrLa) in RO water while stirring. 328mg PVAS-6 was dissolved in 3.9 g RO water and 2.0 g NaOH solution andits pH was adjusted to 6.11, and 550 mg ZrLa (5.5% Zr(IV)] was addedinto the above solution while stirring. The carboxyl groups/Zr(IV) molarratio was 6:1. Zr(IV) loading in Zr-nanogel-43 was 4463 ppm. Detailedinformation regarding Zr-nanogel-43 is listed in Table 1.

Cr-PAsp Nanogel-2 with PolyAspartic acid (PAsp) (Mw=4˜6 k) and CrAc byinverse-emulsion. In order to test nanogels based on other sources ofcarboxylate ions, we made a nanogel with polyaspartate (PAsp) in placeof PVAS. A representative Cr(III)-loaded PAsp nanogel herein referred toas Cr-PAsp nanogel-2 was prepared using PAsp with Cr(III) as CrAc byinverse-emulsion. In such process, an aqueous mixture containing 921 mgPAsp, 232 mg CrAc and 4.6 g NaOH solution as the dispersed phase and anoil mixture of 2.41 g kerosene, 385 mg Span 83 and 217 mg PSH as acontinuous phase were prepared. The inverse-emulsion was prepared bymixing the aqueous phase and the oil phase, followed by rapidhomogenization using a sonicator. The carboxyl groups/Cr(III) molarratio was 7:1. Cr(III) loading in Cr-PAsp nanogel-2 was around 6837 ppm.The mean particle size was around 400 nm. Detailed information regardingCr-PAsp nanogel-2 is listed in Table 1.

Several gelation tests were performed on the various nanogels madeherein to demonstrate the suitability of nanogels containing multivalentcations as crosslinking agents with delayed gelation times. Thefollowing examples show slower gelation rates with these crosslinkerscompared with multivalent cation complexes typically used in gelation ofpartially hydrolyzed polyacrylamides.

Gelation of Cr-Nanogel-27, -28 and -30 with HPAM. In an oxygen-freeglove box, 12.50 g of 2% HPAM solutions were added into 34.17 g ofdeoxygenated Synthetic Brine A in a beaker with stirring. Then 3.33 g ofCr-nanogel-27, Cr-Nanogel-28 or Cr-Nanogel-30 was added into the abovemixture under stirring (final Cr(III) concentration was 100 ppm, finalHPAM concentration was 0.5%). Finally the initial viscosity wasrecorded.

A Brookfield Digital Viscometer Model LVDV-II+PCP was used to monitorthe viscosity changes of gelant and control solutions and determine thegel time of the gelant solutions. The gelation process was monitored asa function of time starting from the point of visual homogeneousdispersion. The gelation time was defined as the time when the viscosityof the gel solution increases abruptly to a value greater than 1000 cP(100% scales) at a shear rate of 2.25 s⁻¹. The temperature of theviscometer was controlled at the stated temperatures during themeasurements.

The composition of Synthetic Brine A used in gelation experiments islisted in Table 2. A second Brine B composition used in laterexperiments is listed in Table 3. The various solutions were thendivided into 6 ml vials and incubated at the indicated temperature(s).The viscosities of the samples were monitored as a function of agingtime.

TABLE 2 Composition of Synthetic Brine A Component Concentration, g/kgNaCl 22.982 KCl 0.151 CaCl₂•2H₂O 0.253 MgCl₂•6H₂O 1.071 NaHCO₃ 2.706Na₂SO₄ 0.145 Water To 1000 g pH 8

TABLE 3 Composition of Synthetic Brine B Component Concentration, g/kgNaCl 18.420 KCl 0.424 CaCl₂•2H₂O 0.550 MgCl₂•6H₂O 0.586 SrCl₂•6H₂O 0.061NaHCO₃ 3.167 Na₂SO₄ 0.163 Water To 1000 g pH 8

The results are shown in FIG. 5. As this figure shows, the delayedrelease of Cr(III) gelation agent from Cr-nanogel-27, -28 and -30produced gels with HPAM is at a much slower rate than the prior artcomplexed multivalent cations used alone to gel HPAM.

Additionally, the highest carboxyl/Cr(III) ratio (6:1) held the Cr(III)tighter and gelled slower with HPAM. The other two ratios of 4.5 and 3thus probably release Cr(III) easier, allowing more rapid gelation withHPAM. Thus, one way the gel time can be increased is by increasing thenumber of carboxylate groups in the nanogel.

Gelation of Cr-Nanogel-29 with HPAM. In an oxygen-free glove box, 12.50g of 2% HPAM solution was added into 34.17 g of deoxygenated SyntheticBrine A in a beaker with stirring. Then 3.33 g of Cr-nanogel-29containing CrAc and PVA succinate 6 k was added into the above mixtureunder stirring (final Cr(III) concentration was 100 ppm, final HPAMconcentration was 0.5%). Finally, the initial viscosity was recorded.The solution was then divided into 6 ml vials and incubated at 85° C.The viscosities of the samples were monitored as a function of agingtime.

The results are shown in FIG. 6, which compares Cr-nanogel-29(PVAS-6-6:1) and Cr-nanogel-27 (PVAS-25-6:1). As this figure shows, thedelayed release Cr(III) gelation agent forms gels with HPAM at a muchslower rate than the prior art complexed multivalent cations used aloneto gel HPAM, which took only hours. However, the two nanogels made withdifferent molecular weight PVAS took about the same time to gel,indicating that the molecular weight of the polymer used to make thenanogel is not a significant factor in delay time, at least under theseconditions.

Gelation of Cr-Nanogel-32 with HPAM. In an oxygen-free glove box, 2.08 g30% inverting surfactant was dissolved in 106.27 g of deoxygenatedSynthetic Brine A in a beaker with stirring. Then 4.15 g Cr-Nanogel-32and 37.50 g of 2% HPAM were added into the above mixture under stirring(final Cr(III) concentration was 100 ppm, final HPAM concentration was0.5%). Finally the initial viscosity was recorded. The solution was thendivided into 6 ml vials and incubated at 65 and 85° C. The viscositiesof the samples were monitored as a function of aging time. The resultsare shown in FIG. 7. As this figure shows, the lower temperature helpedto greatly delay gel times for the Cr-Nanogel-32 (PVAS-6-6:1, 400 nm)from 10 days at 85° C. to about 80 days at 65° C.

Gelation of Cr-Nanogel-33 with HPAM. In an oxygen-free glove box, 1.73 g30% inverting surfactant was dissolved in 88.56 g of deoxygenatedSynthetic Brine A in a beaker with stirring. 3.46 g Cr-nanogel-33 25 kand 31.25 g of 2% HPAM were added into the above mixture under stirring(final Cr(III) concentration was 100 ppm, final HPAM concentration was0.5%), and then the initial viscosity was recorded. The solution wasthen divided into 6 ml vials and incubated at 65 and 85° C. Theviscosities of the samples were monitored as a function of aging time.The results are shown in FIG. 8. The lower temperature delayed gel time,from 5 days at 85° C. to 65 days at 65° C. using this Cr-nanogel-33(PVAS-25-6:1, 400 nm).

Gelation of Cr-Nanogel-32 with B29. We also sought to confirm that thedelayed gelling effect was general, not limited to HPAM polymers. B29 isan expandable microparticle made in part with labile crosslinkers andwith stable crosslinkers. The degree of polymerization is quite high,resulting in a very small microparticle that can be easily pumped andpenetrate the fine pores of the reservoir. Once there, the hightemperature and/or pH results in loss of the labile crosslinker bondsand the remaining polymer absorbs water, swelling greatly in situ. Whileviscous, these polymers are still subject to washout, and thus furthercrosslinking in situ is desirable. We therefore sought to determine ifour delayed gelling agents could also be used with such microparticles.

In an oxygen-free glove box, 2.22 g 30% inverting surfactant wasdissolved in 93.34 g of deoxygenated Synthetic Brine A in a beaker withstirring. Then 2.77 g Cr-nanogel-32 and 1.67 g 30% B29 were added intothe above mixture under stirring (final Cr(III) concentration was 100ppm, final B29 concentration was 0.5%) and finally the initial viscositywas recorded. The solution was then divided into 6 ml vials andincubated at 65 and 85° C. The viscosities of the samples were monitoredas a function of time.

The results are shown in FIG. 9. The delayed release of Cr(III) fromCr-nanogel-32 (PVAS-6-6:1-400 nm) and slow popping of B-29 polymericmicroparticles releasing HPAM results in slower gel formation. Delayranged from 7 days at 85° C. to 80 days at 65° C. B29 is largely thesame as HPAM once it is popped, but its degree of hydrolysis is a bitlower (5%), thus it gels a little slower.

Gelation of Cr-Nanogel-33 with B29. In an oxygen-free glove box, 2.23 g30% inverting surfactant was dissolved in 93.32 g of deoxygenatedSynthetic Brine A in a beaker with stirring, and then 2.78 gCr-nanogel-33 and 1.67 g of 30% B29 were added into the above mixtureunder stirring (final Cr(III) concentration was 100 ppm, final B29concentration was 0.5%). Finally the initial viscosity was recorded. Thesolution was then divided into 6 ml vials and incubated at 65 and 85° C.The viscosities of the samples were monitored as a function of agingtime and results are shown in FIG. 10. As shown, the delay times forCr-nanogel-33 (PVAS-25-6:1, 400 nm) were somewhat reduced as comparedwith Cr-nanogel-32 (PVAS-6-6:1-400 nm) from about 5 days at 85° C. toabout 35 days at 65° C. While preparing Cr-Nanogel-33, we dissolvedPVAS-25 in NaOH solution, because it was difficult to dissolve it inwater, before adding the CrAc. Thus, the NaOH probably accelerated Crrelease from the nanogel.

Gelation of Zr-Nanogel-43 with HPAM. In an oxygen-free glove box, 25 gof 2% HPAM solutions were added into 72.29 g of deoxygenated SyntheticBrine A without NaHCO₃ in a beaker with stirring. Then 2.71 g ofZr-nanogel-43 containing ZrLa and PVAS-6-6:1 was added into the abovemixture under stirring (final Zr(IV) concentration was 120 ppm, finalHPAM concentration was 0.5%), and the initial viscosity recorded. Thesolution was then divided into 6 ml vials and incubated at 85° C. Theviscosities of the samples were monitored as a function of aging time.The results are shown in FIG. 11. As this figure shows, the delayedrelease of Zr(IV) results in gel formation with HPAM at a much slowerrate than the prior art complexed multivalent cations used alone to gelHPAM. The lower temperature helped to greatly delay gel times for theZr-Nanogel-43 from 2 to ˜5 days at 88° C. to around three weeks at 65°C.

Gelation of Cr-PAsp Nanogel-2 with HPAM. In an oxygen-free glove box, 50g of 1 HPAM solutions in Synthetic Brine B were added into 47.81 g ofdeoxygenated Synthetic Brine B with 0.73 g 30% inverting surfactant in abeaker with stirring. Then 1.46 g of Cr-PAsp nanogel-2 containing CrAcand PAsp was added into the above mixture under stirring (final Cr(III)concentration was 100 ppm, final HPAM concentration was 0.5%), and theinitial viscosity recorded. The solution was then divided into 6 mlvials and incubated at 88° C. The viscosities of the samples weremonitored as a function of aging time. The results are shown in FIG. 12.As shows, the delayed release of Cr(III) results in gel formation withHPAM at a much slower rate than the prior art complexed multivalentcations used alone to gel HPAM. The lower temperature helped to greatlydelay gel times for the Cr-PAsp nanogel-2 from 10 days at 106° C. to 34days at 88° C. Also, Cr-PAsp nanogel-2 with HPAM had much longergelation delay than all PVA succinate nanogels, because PAsp hydrolyzedmuch slower than PVA succinate, probably due to greater stability of theamide bonds over ester bonds. Based on these results, we predict thatpolyglutamate, and other polymers having pendant carboxylates and amidebonds should produce a long gel delay time.

Gelation of Cr-PAsp Nanogel-2 with B29. In an oxygen-free glove box,1.67 g of 30% B29 were added into 95.3 g of deoxygenated Synthetic BrineB with 1.57 g 30% inverting surfactant in a beaker with stirring. Then1.46 g of Cr-PAsp nanogel-2 was added into the above mixture understirring (final Cr(III) concentration was 100 ppm, final B29concentration was 0.5%), and the initial viscosity recorded. Thesolution was then divided into 6 ml vials and incubated at 88° C. or106° C., viscosities were monitored and results are shown in FIG. 13.The delayed release of Cr(III) results in gel formation with HPAM at amuch slower rate than the prior art complexed multivalent cations usedalone to gel HPAM. The lower temperature helped to greatly delay geltimes for the Cr-PAsp nanogel-2 from 8 days at 106° C. to 28 days at 88°C. Also, Cr-PAsp nanogel-2 with B29 had much longer gelation delay thanall PVA succinate nanogels, because PAsp hydrolyzed much slower than PVAsuccinate, and this effect is independent of the injection polymer used.

Preparation of CrCl₃-PVAS and CrCl₃—PAsp nanogels using CrCl₃ as Cr(III)source. A representative Cr(III)-loaded PVAS nanogel herein referred toas CrCl₃-PVAS was prepared through mixing PVAS-6 with Cr(III) as CrCl₃in Reverse Osmosis (RO) water while stirring. 573 mg PVAS-6 wasdissolved in 11.44 g RO water with 0.60 g of 10.19% NaOH and 3.42 gCr(III) solution (8761 ppm Cr(III)) was added into the above solutionwhile stirring. The carboxyl groups/Cr(III) molar ratio is 6:1. Cr(III)loading in CrCl₃-PVAS was around 1869 ppm.

CrCl₃-PAsp nanogel was prepared using the same procedure. Arepresentative Cr(III)-loaded PAsp nanogel herein referred to asCrCl₃-PAsp was prepared through mixing PAsp with Cr(III) as CrCl₃ inReverse Osmosis (RO) water while stirring. 653 mg PAsp was dissolved in7.88 g RO water with 3.53 g of 10.19% NaOH and after pH was adjusted to7.63 by addition of 2.57 g 1N HCl, 5.45 g Cr(III) solution (9036 ppmCr(III)) was added into the above solution while stirring. The carboxylgroups/Cr(III) molar ratio is 6:1. Cr(III) loading in CrCl₃-PAsp wasaround 2452 ppm.

Gelation of CrCl₃-PVAS with B29. In an oxygen-free glove box, 0.83 g 30%inverting surfactant was dissolved in 92.15 g of deoxygenated SyntheticBrine A in a beaker with stirring. 5.35 g CrCl₃-PVAS and 1.67 g of 30%B29 were added into the above mixture while stirring (final Cr(III)concentration was 100 ppm, final B29 concentration was 0.5%) and initialviscosity recorded. The solution was then divided into 6 ml vials andincubated at 65 and 85° C., and viscosities monitored as a function oftime. FIG. 14 shows viscosity versus aging time for 0.5% B29 with 100ppm Cr(III) as CrCl₃-PVAS in Synthetic Brine A at 65 and 85° C. Whilethe gelant aged at 65° C. took over around 62 days of aging to begingelling the gelant aged at 85° C. began to gel in about 9 days of aging.FIG. 15 shows viscosity versus aging time for 0.5% B29 with 100 ppmCr(III) as CrCl₃-PAsp (CrCl₃-PAsp-1 and CrCl₃-PAsp-2 are the sameformulation, used to prove the reproducibility of gelation delay) inSynthetic Brine A at 100 and 120° C. While the gelant aged at 120° C.took about 1 day of aging to begin gelling the gelant aged at 100° C.began to gel in about 3-4 days of aging.

Gelation of CrCl₃-PAsp with B29. In an oxygen-free glove box, 0.83 g 30%inverting surfactant was dissolved in 93.42 g of deoxygenated SyntheticBrine A in a beaker with stirring. Then, 4.08 g CrCl₃-PAsp and 1.67 g30% B29 were added into the above mixture while stirring (final Cr(III)concentration was 100 ppm, final B29 concentration was 0.5%) and initialviscosity was recorded. The solution was then divided into 6 ml vialsand incubated at 85, 100 and 120° C. The viscosities of the samples weremonitored as a function of time. FIG. 16 shows viscosity versus agingtime for 0.5% B29 with 100 ppm Cr(III) as CrCl₃-PAsp (CrCl₃-PAsp-1 andCrCl₃-PAsp-2 are the same formulation, used to prove the reproducibilityof gelation delay) in Synthetic Brine A at 85° C. The gelant took overaround 39 days of aging to begin gelling aged at 85° C.

Gelation of CrCl₃-PAsp with HPAM. In an oxygen-free glove box, 100 g of1% HPAM solution in Synthetic Brine A was added into 91.84 g ofdeoxygenated Synthetic Brine A in a beaker with stirring. Then 8.16 g ofCrCl₃-PAsp was added into the above mixture under stirring (finalCr(III) concentration was 100 ppm, final HPAM concentration was 0.5%)and initial viscosity was recorded. The solution was then divided into 6ml vials and incubated at 100 and 120° C. The viscosities of the sampleswere monitored as a function of aging time. FIG. 17 shows viscosityversus aging time for 0.5% HPAM with 100 ppm Cr(III) as CrCl₃-PAsp(CrCl₃-PAsp-1 and CrCl₃-PAsp-2 are the same when aged at the sametemperature. These results indicate the reproducibility of gelationdelay in Synthetic Brine A at 100 and 120° C. While the gelant aged at120° C. took about 1 day of aging to begin gelling the gelant aged at100° C. began to gel in about 2-3 days of aging. FIG. 18 shows viscosityversus aging time for 0.5% HPAM with 100 ppm Cr(III) as CrCl₃-PAsp(CrCl₃-PAsp-1 and CrCl₃-PAsp-2 are the same formulation, used to provethe reproducibility of gelation delay) in Synthetic Brine A at 85° C.The gelant took over around 30 days of aging to begin gelling aged at85° C.

The following references are incorporated by reference in their entiretyfor all purposes.

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1. A delayed gelling composition comprising: a. a degradable compositioncomprising a carboxyl group containing polymer complexed with amultivalent metal ion comprising chromium, zirconium, iron, aluminum,titanium or combinations thereof, wherein said degradable compositionlasts at least 10 days at 85° C. in a brine solution having 23 g/l NaCl,and thereafter degrades to release said multivalent metal ion; b. aninjection fluid; and c. a swellable microparticle copolymer ofacrylamide and sodium acrylate crosslinked with a labile crosslinker andwith a stable crosslinker.
 2. The delayed gelling composition of claim1, said carboxyl group containing polymer selected from polyaspartate(PASP), polyglutamate (PGLU), polyvinyl alcohol succinate (PVAS), orpolyvinyl alcohol malate (PVAM).
 3. The delayed gelling composition ofclaim 1, said carboxyl group containing polymer selected frompolyaspartate (PASP) or polyvinyl alcohol succinate (PVAS) and saidmultivalent metal ion comprising chromium or zirconium.
 4. The delayedgelling composition of claim 1, said multivalent metal ion comprisingchromium or zirconium.
 5. The delayed gelling composition of claim 1,said labile crosslinker being diacrylate and said stable crosslinkerbeing polyethlyene glycol.
 6. The delayed gelling composition of claim5, said multivalent metal ion comprising chromium or zirconium.
 7. Thedelayed gelling composition of claim 1, said multivalent metal ion beingat 50-5000 ppm on a weight/weight basis of said delayed gellingcomposition.
 8. The delayed gelling composition of claim 1, a carboxylgroup to multivalent metal ion ratio being 6:1.
 9. The delayed gellingcomposition of claim 1, said swellable microparticle copolymer being at0.5% concentration on a weight/weight basis of said delayed gellingcomposition.
 10. A method of sweeping for oil or gas, said methodcomprising injecting the delayed gelling composition of claim 1 into areservoir, aging said composition until a viscosity of said delayedgelling composition increases to at least 1000 cP, and thereaftersweeping the reservoir for oil or gas and producing said oil or gas. 11.The method of claim 10, said carboxyl group containing polymer selectedfrom polyaspartate (PASP) or polyglutamate (PGLU) or polyvinyl alcoholsuccinate (PVAS) or polyvinyl alcohol malate (PVAM).
 12. The method ofclaim 10, said multivalent metal ion comprising chromium or zirconium.13. The method of claim 10, said labile crosslinker being diacrylate andsaid stable crosslinker being polyethlyene glycol.
 14. The method ofclaim 10, said multivalent metal ion being at 50-5000 ppm on aweight/weight basis of said delayed gelling composition.
 15. The methodof claim 10, a carboxyl group to multivalent metal ion ratio being 6:1.16. The method of claim 10, said swellable microparticle copolymer beingat 0.5% concentration on a weight/weight basis of said delayed gellingcomposition.
 17. A method of sweeping for oil or gas, said methodcomprising injecting a delayed gelling composition into a reservoir,aging said composition until the viscosity increases to at least 1000cP, and thereafter sweeping the reservoir for oil or gas and producingsaid oil or gas, said delayed gelling composition comprising: a. adegradable nanogel comprising a carboxyl group containing polymerselected from polyaspartate (PASP) or polyglutamate (PGLU) or polyvinylalcohol succinate (PVAS) or polyvinyl alcohol malate (PVAM) complexedwith a multivalent metal ion comprising chromium, zirconium, iron,aluminum, titanium or combinations thereof, wherein said degradablenanogel lasts at least 5 days at 85° C. in a brine solution having 23g/l NaCl, and thereafter degrades and releases said multivalent metalion; b. an injection fluid; and c. a swellable microparticle copolymerof acrylamide and sodium acrylate crosslinked with a labile crosslinkerand with a stable crosslinker, said labile crosslinker being diacrylateand said stable crosslinker being polyethlyene glycol.
 18. The method ofclaim 17, a carboxyl group to multivalent metal ion ratio being 6:1. 19.The method of claim 17, said multivalent metal ion being at 50-5000 ppmon a weight/weight basis of said delayed gelling composition.
 20. Themethod of claim 17, said swellable microparticle copolymer being at 0.5%concentration on a weight/weight basis of said delayed gellingcomposition.